Apparatus of this general type is disclosed in German patent documents DE-AS-33 41 234 and DE-PA 35 38 353. In such apparatus, it is primarily during the heating-up phase that accuracy of measurement is most significantly influenced by changes in the distance of the pick-up from the side of the vessel. The position of the vessel on the hot plate can vary greatly, with the result that the detected surface is warmed more or less quickly.
Such measurement error during heating, caused by the position of the vessel, is compensated for by having the radiation measuring field point with its middle axis at an acute angle to the surface of the hot plate on which the vessel is placed. Such angled arrangement of the radiation pick-up has the result that, with increasing distance between vessel and pick-up, the center of the area of radiation detection moves farther and farther away from the bottom of the vessel. With decreasing distance, the center of the area of radiation detection shifts in a direction toward the bottom of the vessel.
This results in a compensation for the lag in temperature measurements during the heating-up phase by shifting the area of radiation detection closer to the center of the hot plate when the vessel is close to the pick up. If the vessel is moved away from the pick-up, then the faster increases of temperature readings during the heating-up phase, which would take place with a horizontally arranged radiation measurement field, are eliminated by relocation of the area of radiation detection into colder ranges of the side of the vessel, that is, farther away from the bottom of the vessel.
In this way, the measuring errors during the heating-up phase which result from the positioning of the vessel on the hot plate are automatically compensated for. The sensed temperature thus substantially corresponds to the actual temperature of the substance to be heated, regardless of the position of the vessel on the hot plate, that is, regardless of the distance from the pick-up. This is particularly important during the heating-up phase, during which the temperature changes continuously and the temperature level is set.
It has been observed that, in the steady state of such heating systems, the measurement of temperature by radiation sensing is affected in various ways by secondary radiation caused by the temperature of the hot plate itself. In such situations the hot place causes spurious radiation which is dependent upon the position of the cooking vessel on the hot plate. Since the detected radiation intensity decreases as a square of increasing distance, the measurement will be in error more or less, depending upon the share of spurious radiation reaching the pick-up.